Micro-Arc Assisted Electroless Plating Methods

ABSTRACT

A method for electorless plating of a substrate such as magnesium, aluminium, titanium or an alloy, comprises the steps of forming a very thin film of oxide on the substrate by plasma electrolytic oxidation before depositing a layer comprising nickel on the substrate by electroless nickel deposition.

FIELD OF THE INVENTION

The invention relates to electroless plating methods which involve apre-step of plasma electrolytic oxidation.

BACKGROUND TO THE INVENTION

As the lightest structural metal materials, magnesium (Mg) and Mg alloysare finding increasing application in various industries because of anumber of desirable properties. These include high specific strength andstiffness, and excellent castability, machinability and dampingproperties. The driving force also lies in the greatly improvedaffordability of commercial Mg alloys. Thus Mg and Mg alloys exhibitgreat promise. However, the high chemical reactivity of Mg results inpoor corrosion resistance. This is one of the main obstacles to theapplications of Mg alloys in practical environments. Thus providing aprotective surface treatment is an essential part of the manufacturingprocess for many Mg components.

Among various surface treatment techniques, electroless nickel (EN)plating is of particular interest. It has advantages such as uniformdeposition, good corrosion and wear resistance, good electrical andthermal conductivity, and good solderability. Electroless nickel (Ni)coatings on steels, Mg alloys, aluminium (Al) and copper (Cu) have beeninvestigated during the past few years. Previous studies of electrolessnickel coatings on Mg alloys, such as the Dow method, DeLong et al,described in U.S. Pat. No. 3,152,009 use basic nickel carbonate as themain salt in order to minimize the corrosion tendency of the Mg alloysubstrate in the plating bath. This corrosion results in high cost andlow efficiency—see Fatigue properties of Keronite coatings on amagnesium alloy (Surface and Coatings Technology, 2004. 182(1): p.78-84). Gu, C., et al have looked at the use of nickel sulphatesolution—see Electroless Ni—P plating on AZ91D magnesium alloy from asulfate solution (Journal of Alloys and Compounds, 2005. 391(1 2): p.104-109). However, the prior processing methods generally employ CrO₃,cyanide and/or hydrofluoric acid during pre-treatment steps for thesemethods. Such pre-treatment steps are harmful to the operators andunfriendly to the environment. Moreover, galvanic corrosion between theNi coating and the substrate is always a concern, especially when poresexist in the coating. Therefore, developing low cost andenvironment-friendly EN plating on metals and alloys such as Mg and Mgalloys with high-performance is an important task.

The foregoing discussion has been included here for the purpose ofproviding a context for the invention. It is not to be taken as anadmission that any or all of these matters form part of the prior artbase or were common general knowledge in the field relevant to theinvention as it existed before the priority date.

OBJECT OF THE INVENTION

It is an object of the present invention to provide an electrolessnickel process which overcomes or at least ameliorates some of theabovementioned disadvantages or which at least provides the public witha useful choice.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodfor electroless plating of a substrate comprising the steps of forming alayer of oxide on the substrate by plasma electrolytic oxidation (PEO),and depositing a layer comprising nickel on the substrate by electrolessnickel (EN) deposition.

Preferably the PEO step of comprises or results in formation of a verythin layer of dense oxide.

Preferably the substrate is selected from magnesium, aluminium,titanium, copper and their alloys, and iron alloys.

Preferably the PEO step is ceased when the voltage reaches 450V,preferably with current density varying from 0 to 1000 A/m⁻².

Preferably or alternatively the PEO step is ceased when a high densityof nucleation sites are formed.

Preferably between the PEO step and the EN step there is the step ofchemical deposition of palladium onto the substrate.

Preferably after the step of chemical deposition of palladium, and priorto the EN step there a step of reduction of palladium ions to palladiummetal in the vicinity or on the substrate.

Preferably prior to the PEO step there is a pre-treatment step ofpolishing the substrate.

Preferably before or after any or all of the steps of polishing, PEO,EN, chemical deposition of palladium and reduction of palladium ions thesubstrate is washed or cleaned with water.

Preferably the EN is carried out by contacting the substrate with a bathcontaining nickel ions. Preferably the bath includes nickel ions andphosphorous ions. Preferably the pH of the bath is between 6-8 and mostpreferably the temperature is around 80° C.

According to a further aspect of the invention there is provided amethod of preparing a plated substrate prepared substantially accordingto the above method.

Other aspects of the invention may become apparent from the followingdescription which is given by way of example only and with reference tothe accompanying drawings.

As used herein the term “and/or” means “and” or “or”, or both.

As used herein “(s)” following a noun means the plural and/or singularforms of the noun.

The term “comprising” as used in this specification means “consisting atleast in part of”, that is to say when interpreting independentparagraphs including that term, the features prefaced by that term ineach paragraph will need to be present but other features can also bepresent.

To those skilled in the art to which the invention relates, many changesin construction and widely differing embodiments and applications of theinvention will suggest themselves without departing from the scope ofthe invention as defined in the appended claims. The disclosures and thedescriptions herein are purely illustrative and are not intended to bein any sense limiting

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only and withreference to the drawings in which:

FIG. 1: shows a typical PEO experimental set up

FIG. 2: shows a typical EN experimental set up,

FIG. 3: illustrates potentiodynamic polarization curves for AZ91 alloy,PEO coating and PEO+EN coatings of the invention.

FIG. 4: illustrates potentiodynamic polarization curves for AZ91 alloy,traditional EN and PEO+EN coatings of the invention.

FIG. 5: shows micrographs of surface morphology of (a) AZ91 alloy and(b) PEO coated AZ91 alloy after potentiodynamic polarization tests in3.5 wt. % NaCl solution.

FIG. 6: shows micrographs of specimens after 168 h of neutral salt spraytest (NSST) for (a) AZ91 alloy and PEO coating, (b) traditional EN andPEO+EN coatings, the latter of the invention.

FIG. 7: shows SEM images of sample surface for: (a) Dow methodpretreated, (b) PEO pretreated, (c) Dow method EN coating for 4 min, and(d) PEO+EN coating for 4 min, (e) Dow method EN coating for 60 min, (f)PEO+EN for 60 min

FIG. 8: shows the cross-sectional morphology of (a) Dow method ENcoating and (b) PEO+EN coating of the invention.

FIG. 9: presents plots of EDS chemical analysis across the interfacefor: (a) traditional EN coating, and (b) PEO+EN coating of theinvention.

FIG. 10: presents a friction plot for the scratch tests of conventionalEN on AZ91 alloy substrate.

FIG. 11: shows a micrograph of a scratch track of an adhesion test on aconventional EN coating.

FIG. 12: presents a friction plot for scratch tests of the EN coatingprepared by the method of the invention on AZ91 alloy substrate;

FIG. 13: shows a micrograph of a scratch track of an adhesion test on aEN coating of the invention.

FIG. 14: shows micrographs of the ends of scratch tracks on EN coatings:a) conventional EN coating and b) an EN coating of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to providing resistant coatings for metal or metalalloys such as, but not limited to magnesium and magnesium alloys.Coatings may also be formed on titanium, aluminium, copper, and iron andtheir alloys.

Coatings which are successful in protecting a metal from corrosion areideally uniform, well adhered, pore-free, and have a self-healingability.

In accordance with the invention the invention, prior to carrying outelectroless nickel (EN) plating or deposition on a metal or metal alloysubstrate there is a pre-step of plasma electrolytic oxidation (PEO). Wehave found that a plated surface produced according to the method of theinvention exhibits superior characteristics and/or properties to thoseprepared by conventional EN plating methods. In a preferred embodimentof the invention these characteristics and/or properties may include oneor more of:

-   -   the film between the nickel coating and the substrate prepared        by PEO acting as an effective barrier layer and giving rise to        enhanced corrosion resistance of the ultimate coating,    -   decrease of the corrosion current density. For example, the        corrosion current density of the PEO+EN plating on the magnesium        alloy AZ91 as indicated by potentiodynamic tests decreased by        almost two orders of magnitude compared to the traditional EN        coating. Salt fog spray testing further proved this improvement,    -   the absence of chromium species, cyanide or hydrofluoric acid.        The method of the invention does not require the use Cr⁶⁺,        cyanide or HF in its pretreatment, and therefore is a more        environmentally friendly process.

Important steps of the method of the invention comprise: plasmaelectrolytic oxidation, and electroless nickel plating.

Plasma Electrolytic Oxidation

Plasma electrolytic oxidation (PEO) or deposition has been used in theprior art to prepare coatings on Fe, Al, Ti and Mg metal and/or alloys.PEO is a spark-anodizing oxidation method. It involves the modificationof a conventionally anodically grown film by the application of anelectric field greater than the dielectric breakdown field for theoxide. Discharges occur and the resulting plasma-chemical reactionscontribute to the growth of the coating. The technique is friendly tothe environment because no chromate solution is needed.

Conventional PEO involves the formation of a dense and thick ceramiccoating. This ceramic coating is directly used to provide protection tothe substrate alloys. Therefore, porosity in the coatings will greatlyharm the protective ability of the coatings.

In the method of the invention PEO is carried out to produce a very thinlayer of preferably dense and preferably continuous oxide. Without beingbound by any particular theory we believe the oxide of the PEO stepprovides high density of nucleation sites for the next stage electrolesscoating.

A typical PEO set up is illustrated in FIG. 1 and comprises power supply1, working electrodes 2 and electrolyte 3. This is in a bipolarmicro-arc oxidation treatment mode where both electrodes are thesubstrates to be treated. No counter electrode is requited.

The degree of PEO sufficient to be effective will depend upon thesubstrate and the conditions however, for Mg and Mg alloys the voltageusually progresses from low to high, 0 to 450 V; current density from 0to 1000 A/m² over 2 to 10 minutes. This will also apply for many typesof electrolyte which can produce a dense, continuous protective layer onsubstrates such as aluminium, titanium and steels.

We use the applied voltage to decide when to terminate the PEO process.In a preferred embodiment of the invention on Mg or Mg alloys, using theelectrolyte as listed in Table 2 we believe when the voltage reaches˜450 V, there will be enough nucleation sites for the subsequent ENtreatment.

In general the preferred conditions for the PEO process include one ormore of the following:

-   -   the electrolyte is an alkaline-based solution,    -   the concentration for each chemical is no more than 5 g per        litre (very dilute), and    -   the current density should be kept in the region up to 1000        A/m², and ideally not exceeding 1000 A/m², for at least 2        minutes.    -   The electrolyte must be kept under 45° C.,

There are no special requirements concerning the nature of theelectrodes in the PEO process except the electrodes need to beconductive and the surface should be kept clean. Example electrodesinclude Al, Ti and Mg and Fe. It is their ability to form oxide coatingswhich makes them suitable.

In one form of the invention single oxides (eg Al₂O₃, TiO₂ or MgO) willbe formed, but it is possible that mixed oxides will be produced on thesubstrate surface. For example the use of a set of phosphate basedalkaline electrolytes plus additives can produce mixed oxide coatings.For example sodium hydroxide 2-5 g, sodium phosphate 2-5 g, and sodiumsilicate 0.5-1 g, produce magnesium oxide content and a small amount ofhydroxides and silicates. Some hydroxide and oxide may benefit thesubsequent nickel nucleation.

Our preferred conditions are listed in Stage 1 of Table 2.

Electroless Nickel Plating

Electroless nickel (EN) plating is a chemical reduction process whichdepends upon the catalytic reduction of Ni ions in an aqueous solution(containing a chemical reducing agent) and the subsequent deposition ofNi—P alloys or Ni—B alloys without the use of electrical energy.

Ni—P alloy coatings are more common in the art. The deposits typicallycontain P in the range 3-13% by weight.

Ni—B alloy coatings are more common in industrial wear applications fortheir “as-plated” hardness, which is higher than Ni—P.

“Poly” alloys are a combination of Ni, B or P and other metals such asCo, Fe, W, Re or Mo.

FIG. 2 illustrates a typical EN set up comprising a heating element 4,the substrate to be coated 5, electrolyte 6, and an agitator or stirrer7.

In the method of the invention suitable electrolytes may include allkinds of traditional alkaline and acidic based electroless nickelplating electrolytes as is well known in the art. Nickel sulphate orcarbonate are preferred as in nickel salts. A preferred electrolyte isas documented in Stage 4 of Table 1. This will result in a phosphorousconcentration of the final Ni—P coating is 6-10 wt %. However otherphosphorous concentrations are possible, within the scope of theinvention.

General EN process conditions include control of temperature of theelectrolyte, pH of the electrolyte, and chemical concentrations.

A preferred embodiment of the invention has an electrolyte temperatureof around 80° C., and a pH between 6-8. The process is preferablycarried out under ambient atmospheric conditions with some gentlestirring.

TABLE 1 Preferred Plating Process Stages Stage No. Constituent orcondition Value or range 1 PEO NaOH 2-5 g/L NaH₂PO₄•H₂O 3-5 g/L Na₂SiO₃0.5-1 g/L Time 2-10 min Current density 500-1000 A/m² 2 Activation PdCl₂0.2-0.5 g/L HCl 3-5 ml/L Temperature Ambient (298 K) Time 1-2 min 3Reduction NaH₂PO₂•H₂O 30 g/L Temperature Ambient (298 K) Time 0.5-1 min4 Electroless nickel Nickel sulphate 15 g/L plating & operating Citricacid 5 g/L conditions Ammonium bifluoride 10 g/L Sodium hypophosphite 20g/L Ammonium 30 ml/L hydroxide 25% pH (colorimetric) 4.5-6.8 Temperature349-353 K Agitation required Mild mechanicalPreferably there is a water rinse step after each step in this table.

Other Steps

As is illustrated in Table 1 and as described in the description ofexperimental below, other steps may be included in the preferred processof the invention. Whilst not mandatory, these steps are useful forproduct quality and economy reactions (for example steps a) and b)prolong the life of the electrolyte).

a) Pre-Polishing, Prior to PEO

-   -   This is useful for improving the adhesion strength of the oxide        film and the electroless nickel coatings. It also increases the        life of the electrolyte.

b) Cleaning/Washing Steps

-   -   These can be prior to the method and in between each step of the        method if desired. They are again important for improving die        adhesion strength of the oxide film and the electroless nickel        coatings. They can also prolong the life-time of the        electrolyte.

c) Activation and Reduction Steps—Between the PEO and EN Steps

-   -   The activation step again assists in preparation of dense and        effective catalytic nucleation sites for the subsequent        electroless nickel plating. This step involves the chemical        deposition of Pd metal onto the substrate surface.        Well-distributed fine Pd particles are formed on the surface. To        activate the surface other materials such as nickel may be        alternatively used.

Our preferred process may include the use of PdCl₂ to provide the sourceof Pd adatoms for the subsequent catalytic sites.

The reduction step is mainly to control the amount of Pd deposition. Itcan reduce the effect of possible over-doping of Pd ions in theelectroless nickel plating bath or nickel atoms on the substrate (suchas Mg) surface, which will again prolong the lifetime of the bath andimprove the quality of nickel plating. In the preferred process itinvolves the use of sodium hypophosphite to reduce the Pd ions into Pdatoms on the surface.

Experimental

Rectangular specimens of 15×10×2 mm were cut from sand-cast magnesiumAZ91 alloy. The nominal chemical composition of the alloy is given inTable 2. A 2 mm diameter hole was drilled in the middle of one edge ofthe specimens for hanging in the solution during treatments. Thesurfaces of the specimens were mechanically polished by SiC sand papersup to 1200 grit to ensure the same surface toughness, followed bywashing with ethanol in an ultrasonic bath. The operation conditions ofthe PEO treatment and EN plating are shown in Table 1. The initialvoltage for PEO treatment was 80 V DC, after which pulsed DC voltage wasgradually increased with time to a maximum of 430 V at 500 Hz offrequency and 50% duty ratio. For comparison purposes, conventional Dowmethod EN plating also was applied to AZ91 alloy.

The following description of experimental work further illustrates theinvention.

TABLE 2 Chemical composition of AZ91 Mg alloy (wt. %) Al Zn Mn Ni Cu FeSi Ca K Mg 8.9 0.74 0.17 0.001 0.001 <0.001 <0.01 <0.01 <0.01 Bal

In the PEO step the NaOH provides the pH value required and producesmagnesium oxides NaH₂PO₄ helps to produce magnesium phosphate on thesurface. Sodium silicate acts as a corrosion resistant agent for themagnesium alloy.

In the EN step the Nickel sulphate provides the source of nickel ionsfor the nickel plating. A very small amount of HF helps to dissolve thenickel sulphate and acts as corrosion resistant buffer. This smallamount is not detrimental. In other forms of the invention nickelcarbonate is used. The citric acid acts as a complexant (to reduce theplating speed and keep the bath stable), ammonium bifluoride acts asbuffer, sodium hypophosphite is the main reductant, and ammoniumhydroxide is used to adjust the pH value of the electroless nickel bath.

TABLE 3 The other EN plating electrolyte Constituent and Conditions 1.Low P 2. Medium P 3. High P Basic nickel sulfate 10 g/L 10 g/L 10 g/LCitric acid 5 g/L 5 g/L 20 g/L Ammonium bifluoride 10 g/L 10 g/L 10 g/LSodium hypophosphite 10 g/L 20 g/L 40 g/L Ammonium hydroxide 25% 40 ml/L30 ml/L 50 ml/L Thiourea 1 mg/L 1 mg/L 1 mg/L pH (colorimetric) 6.5-7.54.5-6.8 4.5-6.0 Temperature (K) 349-353 349-353 349-353 Agitationrequired Mild Mild Mild

It should be noted that the EN processes are complex, and especially ENcoating on Mg alloys are not fully understood. Therefore our discussionof the roles of the constituents above are to the best of our knowledgeand we do not wish to be bound by this.

To evaluate the effect of the PEO coating, the potentiodynamicpolarization measurement in 3.5 wt. % NaCl solutions was carried out tocompare the corrosion behaviour of the uncoated Mg alloy, PEO coated andPEO+EN coated samples. The polarization range was from −2.0 to +0.2 Vversus saturated calomel electrode. The polarization scanning rate was40 mV/min. The neutral salt spray test (NSST) was also carried out for168 hours according to ASTM B117-97 standard. Specimens were inspecteddaily.

The morphology and microstructure of the deposits were examined by meansof optical microscopy and scanning electron microscopy (SEM, PhilipsXL30S) with a field emission gun. Energy-dispersive spectroscopy (EDS)and XPS were used to analyse the chemical composition and states. Thestructure of the deposits was also determined by using a D8 advancedX-ray diffractometer.

Results and Discussion

Al and Zn are the main alloying elements in AZ91 alloy (Table 2). Thealloy consists of two phases as shown in FIG. 6 a. The α-Mg matrix is aMg—Al—Zn solid solution with the same crystal structure as pure Mg, andthe β precipitates are intermetallic phase Mg₁₇A₁₁₂. This intermetalliccompound, segregated at grain boundaries, has a free corrosion potentialof −1.0 V, while the α-phase has a free corrosion potential of −1.73 V.Therefore, the intermetallic compound would provide some advantages forthe nucleation during the PEO treatment. The PEO coating clearly shows afew peaks of MgO by means of XRD pattern result.

Potentiodynamic polarization testing: FIG. 3 shows potentiodynamicpolarization curves for the uncoated Mg alloy and the specimens with PEOcoating and PEO+EN coating in 3.5 wt. % NaCl solution at roomtemperature. For the Mg alloy and the alloy treated by PEO, anactivated-controlled cathodic process occurred in the cathodic branch,and the main reaction was hydrogen evolution. The surface of thespecimen changed little in the cathodic process. When the appliedpotential increased into the anodic branch, an activation-controlledanodic process was observed. The polarization current increased withincreasing applied anodic potential, and no passivation occurred.Although the intersection point of the anodic and cathodic curve(E_(corr)) showed a little shift to the positive direction, thepolarization current density decreased by two orders of magnitudes underthe same potential for the specimen treated by PEO. This can beexplained as the PEO coating acted as an insulating barrier which canreduce the current density significantly.

For the alloy with the new PEO+EN coating, the cathodic reaction wasstill hydrogen evolution, but an obvious passivation occurred in theanodic branch. The surface morphology of the two types of specimensafter the potentiodynamic polarization is shown in FIG. 5. For theuncoated Mg alloy, the morphology of attack changed from pittingcorrosion to overall corrosion with increasing potential (FIG. 5 a). Forthe alloy with PEO coating, the attack was pitting corrosion, whichgradually increased with the increase of applied potential (FIG. 5 b).But the degree of attack was much lower than that on the bare Mg alloy.Chlorine content was high around the corroded area in FIG. 5 by EDSanalysis, indicating that Cl− is corrosive and associated with MgO andMg(OH)₂. However, for the alloy with PEO+EN coating, the corrosionresistance was greatly improved, and passivation occurred during anodicpolarization. There is no obvious pitting corrosion when the appliedpotential reaches 200 mV.

The electrochemical behaviour of the new coating was also compared witha traditional EN coating on AZ91 (FIG. 4). It can be clearly seen thatthe E_(corr) is almost same, probably due to the two EN coatings havingthe same chemical composition. However, the corrosion current density(i_(corr)) decreased by more than two orders of magnitudes under thesame potential for the specimen with PEO+EN coating, indicating that thenew EN coating has less porosity and better corrosion resistance. It canbe seen from the PEO pretreated surface in FIG. 8 b that the averagepore size is no more than 5 μm, and that there is a more uniform poredistribution than that of the traditional process in FIG. 8 a. Moreover,overall the PEO coating in this work is a denser and thinner oxidizedlayer as compared to the results reported by Yerokhin, A. L.

Salt spray testing: FIG. 6 shows photos of the typical morphologies ofspecimens after 168 h neutral salt spray testing. There is no noticeablegalvanic corrosion pits on the surface of the PEO and PEO+EN coatings,demonstrating that the PEO coating and the new PEO+EN coating havebetter corrosion resistance than that of Mg alloy and the conventionalEN coating. As discussed above, the two phases, α-Mg matrix and theβ-Mg₁₇A₁₁₂, have very different corrosion behaviour. Therefore galvaniccorrosion took place around the phase boundaries as shown in FIG. 6 a.

For the PEO coating, an effective barrier layer was formed, whichprevent the penetrating of the salt solution. From the cross sectionSEM, we can see that the PEO layer is only 5 μm thick. This indicatesthat the PEO layer must be very compact.

SEM, EDS, XPS and XRD were used to characterise the coatings. FIG. 7shows SEM micrographs of the samples at different treatment stages.FIGS. 7 a and 7 b show a AZ91 sample surface pretreated by Dowpre-treatments (7 a) and after PEO pretreatment (7 b). It can be clearlyseen from FIGS. 7 a and 7 b that the traditional treatment does notproduce a uniform and dense protective film before EN plating. Moreimportantly, the PEO processing only requires one-step pre-treatmentrather than three-step operations which are typical of the prior art,and eliminates the use of hazardous chemicals such as chromium acid andhydrofluoric acid. Furthermore, we found that the nucleation mechanismis also different. FIGS. 7 c and 7 d show the surface after conventionalEN coating (4 minutes) (7 c) and after PEO, +EN coating for 4 minutes (7d). The prior art EN coatings have been discussed as beingpreferentially nucleated on the β-phase or in the vicinity, resulting innon-uniform distribution. On the contrary, as is shown in 7 d, thenucleation of the new EN processing is quite uniform on the PEOpre-treated surface, and the nucleation density is higher than that ofthe traditional process. This indicates that the PEO pre-treatmenteliminates the effect of the electrochemically heterogeneous AZ91substrate surface. Consequently, the final surface of the new EN coating(FIG. 7 is smoother and more uniform than that of the Dow EN coating asshown in FIG. 7 e.

Moreover, it can be found that the nodular size in FIG. 7 f is about 10μm or less whereas the nodular size of the traditional EN surface isoften bigger than 50 μm, as shown in FIG. 7 e. The coarse nodularstructure in FIG. 7 e probably contains more pores around the nodularand substrate grain boundaries. Therefore, it can be concluded that thenew PEO+EN coating provides a less porous duplex coating than thetraditional one, and hence reduces the possible galvanic corrosionbetween the Ni coating and Mg substrate significantly.

FIG. 8 shows SEM cross-sectional morphology of two types of EN coatingson AZ91 alloy. It can be seen that the traditional EN process produces atough and heterogeneous interface (8 a) between the EN and the substratedue to the strong etching effect of chromium acid (CrO₃). However theinterface between the new EN and the substrate (8 b) is relativelysmooth, and the coating has a more uniform thickness and smoothersurface, as shown in FIGS. 6 b and 6 f.

FIGS. 9 a and 9 b show the EDS chemical analysis along the white linesin FIGS. 8 a and 8 b. It can be seen that oxygen concentration is lowerthan fluorine around the interface region in FIG. 9 a. It can be seenfrom FIG. 8 b that oxygen content is higher than fluorine and the rangeof oxygen is much wider (5 μm) than that of the traditional EN (˜2 μm,indicating that PEO pre-treatment in the new EN process produces anoxide film of ˜5 μm thick. Moreover, it can also be found that theoxygen concentration gradually increased to ˜25 wt. % around theinterface region in FIG. 9 b, indicating that the PEO technique canproduce gradient coatings.

Scratch adhesion strength: These results are discussed with reference toFIGS. 10 to 14. FIGS. 10 and 12 are plots of the measured friction force(Fx) vs. loading (Fz) for two coatings. FIG. 9 is the plot for thescratch tests of a conventional EN on AZ91 substrate, whilst FIG. 12 isfor an EN coating prepared on AZ91 substrate in accordance with theinvention.

An increasing load was applied via a diamond tip which was moving on thetop surface of the coatings. The measured Fx shows the increasing wearforce with increasing load. The transition point (critical load, Lc)indicates the penetrating of the coating, which can be considered asreflecting the adhesion strength of the coating.

The conventional electroless nickel (EN) on AZ91 Mg alloy shows not onlya tough surface, but also lower adhesion strength (Critical Load,Lc=10.8 N) than that of the novel EN coating (14.6 N) on AZ91.

FIG. 11 shows the scratch track of the adhesion test on the conventionalEN coating whilst FIG. 13 shows the track for the coating of theinvention.

In general these results illustrate that the coating of the inventionshows a higher critical load, and a smoother friction force than theconventional EN coating.

Furthermore, it can be seen from FIG. 14, which illustrates the ends ofscratch tracks on EN coatings (a) conventional EN, (b) the invention EN(the arrows point in the scratching direction), that the failurebehaviour is different for the two coatings, probably due to thedifferent interface structure for the two coatings (a: MgF₂, b: MgO).The scratch track on the EN coating produced by the method of theinvention is narrower than that on the conventional EN. This may beattributed to the different hardness of the interlayer. The PEO film asthe interlayer has a higher hardness than the conventional one.

Experimental Conclusions

Salt fog spray and potentiodynamic polarization testing demonstrate thatthe PEO treatment produces a dense, well adhered oxide coating on theAZ91 Mg alloy. The presence of the PEO film between the nickel platingand the substrate acted as an effective barrier layer, and also providedhigh density nucleation sites after activation treatment for thesubsequent EN coating, which can significantly reduce the porosity ofthe nickel coating. Therefore, the coatings produced via PEO+EN processpossess superior corrosion resistance to salt spray testing as comparedto the traditional EN coatings. Potentiodynamic polarization tests alsoindicated that the corrosion current density of the new coating on AZ91decreased by at least two orders of magnitudes. This new coating processdoes not need Cr⁶⁺ and HF, and is therefore more environmentallyfriendly.

Although the invention has been described by way of example and withreference to particular embodiments, it is to be understood thatmodifications and/or improvements may be made without departing from thescope or spirit of the invention as described in the accompanyingclaims.

1. A method for electroless plating of a substrate comprising the stepsof: forming a layer of oxide on the substrate by plasma electrolyticoxidation (PEO), and depositing a layer comprising nickel on thesubstrate by electroless nickel (EN) deposition.
 2. A method accordingto claim 1 wherein the substrate is selected from magnesium, aluminium,titanium and their alloys, and iron alloys.
 3. A method according toclaim 1 wherein the substrate consists essentially of magnesium, amagnesium alloy, aluminium, an aluminium alloy, titanium, or a titaniumalloy.
 4. A method according to claim 1 wherein the substrate ismagnesium or a magnesium alloy.
 5. A method according to claim 1 whereinthe substrate is copper or a copper alloy.
 6. A method according toclaim 1 comprising carrying out PEO to a voltage between electrodes upto about 450 Volts.
 7. A method according to claim 1 comprising carryingout PEO for a time of at least two minutes.
 8. A method according toclaim 1 comprising carrying out PEO for a time of up to 10 minutes.
 9. Amethod according to claim 1 comprising carrying out PEO with a currentdensity of up to about 1000 A/m⁻².
 10. A method according to claim 1comprising carrying out PEO at an electrolyte temperature at or belowabout 45° C.
 11. A method according to claim 1 comprising carrying outPEO at an concentration of major electrolyte constituents of up to 5g/litre.
 12. A method according to claim 1 comprising carrying out PEOin an electrolyte which includes phosphate ions.
 13. A method accordingto claim 1 comprising carrying out PEO to form said layer of oxide as avery thin film of oxide on the substrate.
 14. A method according toclaim 1 wherein depositing a layer which includes nickel on thesubstrate by EN deposition comprises depositing a layer of substantiallypure nickel metal on the substrate.
 15. A method according to claim 1comprising carrying out EN deposition in an electrolyte which includesnickel sulphate as a nickel source.
 16. A method according to claim 1comprising carrying out EN deposition in an electrolyte which includesnickel carbonate as a nickel source.
 17. A method according to claim 1comprising depositing a layer comprising nickel deposition by EN at anelectrolyte pH of between about 4.5 and about
 8. 18. A method accordingto claim 1 comprising depositing a layer comprising nickel deposition byEN at an electrolyte pH of between about 6 and about
 8. 19. A methodaccording to claim 1 comprising depositing a layer comprising nickel byEN deposition at an electrolyte temperature of about 80° C.
 20. A methodaccording to claim 1 also including chemically depositing metal onto thesubstrate between the PEO and the EN deposition. 21.-23. (canceled) 24.A method for electroless plating of a substrate which consistsessentially of magnesium, a magnesium alloy, aluminium, and aluminiumalloy, titanium or a titanium alloy, comprising the steps of forming avery thin film of oxide on the substrate by plasma electrolyticoxidation (PEO), and depositing a layer comprising nickel on thesubstrate by electroless nickel (EN) deposition in an electrolyte whichincludes nickel sulphate or nickel carbonate as a nickel source. 25.-28.(canceled)